class Model : public QAbstaractItemModel
{
public:
	int x() const;

	// Other QAbstaractItemModel's mandatory methods implementation, like data(), etc.
	// ...
};

class ModelWidget : public QWidget
{
public:
	ModelWidget(Model& model) :
		model_(model)
	{
		treeView_->setModel(&model);
		model.x();
	}

private:
	Model& model_;
	QTreeView* treeView_;
};

class Xable
{
public:
	virtual int x() const = 0;
};

class Model : public QAbstractItemModel, public Xable
{
public:
	// Xable's implementation
	int x() const {...}

	// Other QAbstaractItemModel's mandatory methods implementation, like data(), etc.
	// ...
};

class SortFilterProxyModel : public QSortFilterProxyModel, public Xable
{
public:
	// Xable's implementation
	int x() const
	{
		if (!sourceModel()) return 0;  // return something suitable or throw an exeption

		// We assume only Model as possible source model
		Model* sourceModel = boost::polymorphic_downcast<Model*>(sourceModel());
		return sourceModel->x();
	}

	// Other QSortFilterProxyModel's methods
	// ...
};

class ModelWidget : public QWidget
{
public:
	ModelWidget(QAbstractItemModel& sourceModel) :
		model_(model)
	{
		treeView_->setModel(model);
		modelAsXable().x();
	}

private:
	QAbstractItemModel& model_;
	QTreeView* treeView_;

	Xable& modelAsXable()
	{
		return dynamic_cast<Xable&>(model_);
	}
};

class ModelWidget : public QWidget
{
public:
	ModelWidget(QAbstractItemModel& model) :
		model_(model)
	{
		treeView_->setModel(model);
		modelAsXable().x();
	}

private:
	QAbstractItemModel& model_;
	QTreeView* treeView_;

	Xable& modelAsXable()
	{
		return dynamic_cast<Xable&>(initialSourceModel(model_));
	}
};

/// \file
/// \brief Initial source model
/// \author Akon
/// \date 01.07.2009

/* ##todo:
*/

#pragma once

#include <QtGui/QAbstractProxyModel>

namespace Akon {
namespace QtGui {

/// Returns initial (first) source model of passed proxy model.
inline QAbstractItemModel* initialSourceModel(const QAbstractProxyModel& model)
{
	QAbstractItemModel* result = model.sourceModel();
	while (qobject_cast<QAbstractProxyModel*>(result))
		result = static_cast<QAbstractProxyModel*>(result)->sourceModel();
	return result;
}

/// If \a model is a proxy model returns initial (first) source model; else returns \a model
/// itself as non-const pointer.
inline QAbstractItemModel* initialSourceModel(const QAbstractItemModel& model)
{
	return qobject_cast<const QAbstractProxyModel*>(&model) ?
		initialSourceModel(static_cast<const QAbstractProxyModel&>(model)) :
		const_cast<QAbstractItemModel*>(&model);
}

}  // QtGui::
}  // Akon::

template <class BaseT>
class SortFilterModelAspect : public <BaseT>
{
	// All functional of QSortFilterProxyModel is implemented in this class template
};

Model model;  // source model with x() "non-standard" function

SortFilterModelAspect<Model> proxyModel;  // source model with x() "non-standard" function + proxy features
proxyModel.setFilterRegExp(...);

class ModelWidget : public QWidget
{
public:
	ModelWidget(QAbstractItemModel& model, X& x) :
		model_(model),
		x_(x)
	{
		treeView_->setModel(model);
		x.x();
	}

private:
	QAbstractItemModel& model_;
	X& x_;
	QTreeView* treeView_;
};